static LS ai_dropTokens(LS tok, int i, int j) {
  // move to code tokens
  i |= 1;
  j |= 1;
  if (j <= i) ret tok;
  S punctuation = ai_mergePunctuation(tok.get(i-1), tok.get(j-1));
  LS l = withoutSubList(tok, i, j);
  l.set(i-1, punctuation);
  ret l;
}